As circuit substrates used in power modules and the like, from the viewpoints of thermal conductivity, costs, and safety, ceramic substances such as those of alumina, beryllia, silicon nitride, and aluminum nitride are used. These ceramic substrates are used as circuit substrates wherein metal circuit boards such as copper and aluminum and heat dissipation plates are bonded. Those have excellent insulative properties and heat dissipation compared with rosin substrates and metal substrates using resin layers as insulative materials, so they are used as substrates for loading high heat dissipation electronic components.
Ceramic circuit substrates wherein a metal circuit board is bonded to the surface of a ceramic substrate with a brazing material and a semiconductor element is loaded onto a predetermined position of the metal circuit hoard are used in power module applications such as elevators, vehicles, and hybrid cars. In recent years, with respect to the increase of heat quantity from semiconductor elements accompanying the high integration, frequency, and output of semiconductor elements, ceramic substrates of aluminum nitride sintered bodies and silicon nitride sintered bodies having high thermal conductivity are used. In particular, aluminum nitride substrates, when compared to silicon nitride substrates, have high thermal conductivity, so they are favorable as ceramic circuit substrates for loading high heat dissipation electronic components.
However, aluminum nitride substrates, while having high thermal conductivity, also have low mechanical strength and toughness, so problems such as cracks occurring because of the tightening at the assembly step and the easy formation of cracks when heat cycles are applied remain. In particular, when used in power modules applied under severe loads and thermal conditions such as those for motor vehicles and electric railways and machine tools and robots, such problems become readily evident.
For this reason, as ceramic substrates for loading electronic components, improvements of mechanical reliability are required, so silicon nitride substrates, though inferior to aluminum nitride substrates in terms of thermal conductivity, have been focused on for having excellent mechanical strength and toughness.
Ceramic circuit substrates making use of silicon nitride substrates are manufactured with, for example, the following active metal method.
The active metal method bonds metal plates on a ceramic substrate via a brazing material layer that includes active metals such as 4A group elements and 5A group elements, and normally, screen prints a silver-copper-titanium brazing material on both main surfaces of a silicon nitride substrate, wherein on these printed surfaces, a metal circuit board and as metal heat dissipation plate are disposed, and wherein, by performing a heating treatment at an appropriate temperature, the ceramic substrate and the metal plates are bonded.
With as thus obtained ceramic circuit substrate, since Ti, which is an active metal, covalently bonds with. N of the nitride ceramic substrate to farm TIN (titanium nitride), and a bonding layer is formed with this TiN, it is possible to obtain a relatively high bonding strength.
On the other hand, in semiconductor modules to be equipped on a vehicle, high output and high integration are progressing, and the thermal stress that repeatedly acts on ceramic circuit substrates tends to increase even more. Once this thermal stress cannot be endured, microscopic cracks occur on the ceramic substrate. In cases where heat load cycles continue to be applied with these microscopic, cracks, the metal plates separate from the ceramic substrate, leading to bonding strength insufficiencies or thermal resistance insufficiencies. As a result, there are problems with the operational reliability as an electronic instrument declining. From such issues, the following have been proposed concerning brazing material constitutions for ceramic circuit substrates that can withstand thermal stress.
Patent Document 1, directed to improving heat cycle resistance properties for ceramic circuit substrates, describes the inclusion of carbon powder in the brazing material for bonding a ceramic substrate and metal plates as being effective.    Patent Document 1: JP H9-283656 A